354 CRETACEOUS. 



attended the deposit of the Wealden sediments ; and the two 

 groups of strata have sometimes been classed together. (See p. 346.) 

 The Lower Greensand, Gault, Upper Greensand, and Chalk in- 

 dicate marine conditions of varying character ; '■ and although 

 locally we have indications of transition from the Wealden deposits 

 upwards to the Chalk, yet over large portions of our area the Lower 

 Greensand rests unconformably on the Wealden and Upper Oolitic 

 strata beneath.- (See Fig. 58.) 



In the Wealden area the Gault rests conformably on the Lower 

 Greensand, although, as Mr. Topley remarks, there is a great 

 palseontological break between them.^ In other parts of England, 

 as in Wiltshire, the Gault overlaps the Lower Greensand and rests 

 on older beds ; and this general overlap of the Upper Cretaceous 

 beds over the Lower beds, and their overstep across different 

 members of the Jurassic system, is well marked in the south of 

 England, in the Midland Counties, in Lincolnshire and Yorkshire.* 

 We see, in fact, evidence of a plain of marine denudation, that was 

 commenced during the Lower Greensand period, and continued in 

 Upper Cretaceous times. 



In Yorkshire the Wealden Beds are represented in part by a 

 series of marine clays included under the general name of Speeton 

 Clay. 



The Cretaceous Rocks were mapped for the Geolonjical Survey, in the south 

 and south-west of England, chiefly by W. T. Aveline, H. W. Bristow, H. 

 Bauerman, and W. Whitaker ; in the south-eastern area, by H. W. Bristow, F. 

 Drew, W. Whitaker, C. Gould, C. Le Neve Foster, and W. Topley ; and in the 

 eastern and north-eastern counties, by W. Whitaker, W. H. Penning, C. Fox 

 Strangways, F. J. Bennett, A. J. Jukes-Browne, A. Strahan, and the writer. 



^ See S. V. Wood, Jun. On the Form and Distribution of Land-tracts during 

 the Secondary and Tertiary Periods, Phil. Mag. 1862 ; C. J. A. Meyer, G. Mag. 

 1 866, p. 13 ; J. S. Gardner, Q. J. xl. 122. 



2 See also Fitton, T. G. S. (2), iv. plate 10a, and p. 189 ; and Judd, Q. J. 

 xxvii. 221. 



^ Geology of Weald, p. 3. 



* Conybeare, Phil. Mag. (2), i. 118; Phillips, Geol. Oxford, p. 422; J. W. 

 Judd, Q J. xxiii. 242 ; C. E. De Ranee, G. Mag. 1874, p. 246. 



